This invention relates to a load sensor used in a weigher or the like and, more particularly, to a load sensor so adapted as to sense a load by utilizing elastic surface waves.
An elastic surface-wave oscillator (surface acoustic-wave oscillator) for measuring the magnitude of an unknown applied force by utilizing elastic surface waves (surface acoustic waves) is disclosed in the specification of Japanese Patent Application Laid-Open No. 50-1022. As shown in FIG. 1, this delay line-type elastic surface-wave oscillator generally comprises an elastic surface-wave oscillator which includes a piezoelectric substrate 1 fixed at one end, a transmission transducer 2 composed of interdigital electrodes 2a, 2b disposed on the surface 1a of the substrate 1, a reception transducer 3 spaced a prescribed distance l from the transmission transducer 2 and similarly composed of interdigital electrodes 3a, 3b disposed on the surface 1a of the substrate 1, and an amplifier 4 connected across one electrode 2a of the transducer 2 and one electrode 3a of the transducer 3. The other electrodes 2b, 3b are connected to ground. Owing to such construction, the delay line-type elastic surface-wave oscillator includes a surface wave propagation path I from the transmission transducer 2 to the reception transducer 3 via the substrate surface 1a, and a feedback path II from the reception transducer 3 to the transmission transducer 2 via the amplifier 4.
The oscillator has an oscillation frequency f.sub.o which is a function of the distance l covered by the surface wave propagation path I, and of the velocity v at which the surface wave propagates through the path I. When an external force F acts upon the free end of the substrate 1 as shown for example in the drawing, flexure of the substrate 1 in the manner shown by the phantom lines is accompanied by a change in the distance l, to l+.sub..DELTA. l. Likewise, the propagating velocity changes to v+.sub..DELTA. v in dependence upon a change in the elasticity of the substrate surface 1a. In consequence, oscillation frequency changes to f.sub.o +.sub..DELTA. f.sub.o. Here the change .sub..DELTA. f.sub.o in frequency corresponds to the amount of flexure of the substrate 1, namely to the magnitude of the applied force F. Therefore, if the frequency change .sub..DELTA. f.sub.o is detected, the magnitude of the applied force F can be measured.
To obtain a desired measurement accuracy in a case where an elastic surface-wave oscillator of the above kind is applied to a load sensor, transmission and reception transducers similar to those shown in FIG. 1 are formed on the reverse side of the substrate 1 as well to provide on the reverse side an oscillator identical to that shown. Then, in response to the some applied force, the oscillation frequency of one of the oscillators will change to f.sub.o +.sub..DELTA. f.sub.o, and the oscillation frequency of the other will change to f.sub.o -.sub..DELTA. f.sub.o. The arrangement is such that the difference between these two frequencies, namely 2.sub..DELTA. f.sub.o, is then detected.
Strain gauge-type load sensors have long found wide use as the load sensors in weighers and the like. As shown in FIG. 2, such a load sensor, designated at numeral 7, includes a load-sensitive element 5 constituting the main body of the load sensor and is a body which develops strain when subjected to an external load. The load-sensitive element 5 has a hollow, rectangular configuration and comprises a fixed rigid portion 5a at one extremity of the rectangle, a movable rigid portion 5b at the other extremity of the rectangle, and upper and lower beams 5c, 5d extending in parallel and interconnecting the rigid portions 5c, 5d. The upper beam 5c is provided at two locations with flexible portions 5.sub.1, 5.sub.2 formed by reducing the thickness of the beam. Likewise, the lower beam 6 is provided at two locations with flexible portions 5.sub.3, 5.sub.4 similarly formed by reducing the thickness of the beam. The load-sensitive element 5 thus has a total of four flexible portions 5.sub.1, 5.sub.2, 5.sub.3, 5.sub.4 . Four strain gauges 6.sub.1, 6.sub.2, 6.sub.3, 6.sub.4 are bonded to the outer surface of the load-sensitive element 5 at the locations of the flexible portions 5.sub.1, 5.sub.2, 5.sub.3, 5.sub.4, respectively. The four strain gauges form a bridge circuit. When an external force F of the kind shown in FIG. 2 acts upon the movable rigid portion 5b of the load sensor 7 with the fixed rigid portion 5a thereof being held stationary, the flexible portion 5.sub.1 of the upper beam 5c on the side of the fixed rigid portion 5a, and the flexible portion 5.sub.4 of the lower beam 5d on the side of the movable rigid portion 5b, are subjected to tension, while the two other flexible portions 5.sub.2, 5.sub.3 are subjected to compression. These tensile and compressive forces result in a change in the electrical resistance of the strain gauges 6.sub.1, 6.sub.2, 6.sub.3, 6.sub.4 located at the flexible portions 5.sub.1, 5.sub.2, 5.sub.3, 5.sub.4, respectively, so that the bridge circuit produces an electrical output signal. The signal corresponds to the amount of strain developed at the flexible portions 5.sub.1, 5.sub.2, 5.sub.3, 5.sub.4, namely to the external fource F. This allows the magnitude of the force F to be measured.
A load sensor which operates by using the above-described delay line-type elastic surface wave oscillator can be constructed by utilizing the load-sensitive element 5 of the load sensor 7. Specifically, as shown in FIG. 3, a delay line-type elastic surface-wave oscillatior 9 . . . 9 may be provided in place of each strain gauge on the upper and lower surfaces of a load-sensitive element 8 at flexible portions 8.sub.1, 8.sub.2, 8.sub.3, 8.sub.4 thereof, each resonator 9 comprising a piezoelectric substrate 9a, a transmission transducer 9b, and a reception transducer 9c, the transducers being disposed on the substrate 9a and spaced from each other by a predetermined distance. The result is a load sensor 10 using a delay line-type elastic surface-wave oscillator and equipped with four elastic surface-wave resonators.
Since the conventional load sensor of the above kind has the delay line-type elastic surface-wave oscillators fixed to the upper and lower surfaces of the load-sensitive element, sealing members are affixed to the load-sensitive element to protect the oscillators from the environment. When such a load sensor is used in, say, a weigher, the overall apparatus must be large enough to prevent the sealing members from contacting the weighing dish or the weigher case. The unfortunate result is a weigher which is large in size.
Another problem with the prior-art load sensor is that since the prior art load sensor is a delay line-type oscillator, it is subject to outside adverse effects and a problem arises in the short-term stability of the sensor output signal.
As shown in FIG. 1, the conventional load sensor has the transmission and reception transducers 2, 3 spaced apart by the distance l to form an elastic surface-wave oscillator unit, with a certain period of time being required for a surface wave to propagate between the two transducers 2, 3. Since the load sensor is a delay line-type oscillator, namely one in which propagation time changes in dependence upon the state of an applied force, a problem arises in the short-term stability of the sensor output signal when the surface wave propagation paths pick up dust, foreign matter such as moisture, scratches or are affected by other such external factors.
In addition to the problem of short-term stability of the output signal, there is a problem related to the positional precision of the elastic surface-wave oscillators on the load-sensitive element. Specifically, as shown in FIG. 3, the transmission transducers 9 . . . 9 are disposed at the flexible portions 8.sub.1, 8.sub.2, 8.sub.3, 8.sub.4 of the load-sensitive element 8. Since the amount of tension or compression caused by the load at the flexible portions 8.sub.1, 8.sub.2, 8.sub.3, 8.sub.4 varies greatly with position, a desirable output cannot be obtained if there is even a small error in the positions at which the elastic surface-wave oscillators 9 . . . 9 are disposed.